Heterometallic Triiron-Oxo/Hydroxo Clusters: Effect of Redox-Inactive Metals

A series
of tetranuclear oxo/hydroxo clusters comprised of three
Fe centers and a redox-inactive metal (M) of various charge is reported.
Crystallographic studies show an unprecedented Fe3M­(μ4-O)­(μ2-OH) core that remains intact upon
changing M or the oxidation state of iron. Electrochemical studies
reveal that the reduction potentials (E1/2) span a window of 500 mV and depend upon the Lewis acidity of M.
Using the pKa of the M-aqua complex as
a measure of Lewis acidity, these compounds display a linear dependence
between E1/2 and acidity, with a slope
of ∼70 mV per pKa unit. The current
study of [Fe3MO­(OH)] and previous ones of [Mn3MOn] (n = 2,4) moieties
support the generality of the above relationship between the reduction
potentials of heterometallic oxido clusters and the Lewis acidity
of incorporated cations, as applied to clusters of different redox-active
metals.